Vignetting

Thanks to the sweet spot behavior on the K10D the vignetting is fairly well
controlled for a wide-angle zoom lens. At 20mm @ f/4 you may see the slightly darker
edges in critical scenes but at other settings it shouldn't be really field
relevant anymore.

MTF (resolution)

The Pentax FA 20-35mm f/4 AL produced high resolution figures in the MTF lab.
At 20mm the center resolution is excellent followed by very good
quality borders and good to very good extreme corners. At 35mm the performance
is a bit more even across the frame thanks to improved borders.

Below is a simplified summary of the formal findings. The chart shows line widths
per picture height (LW/PH) which can be taken as a measure for sharpness.
If you want to know more about the MTF50 figures you may check out the corresponding
Imatest Explanations

Chromatic Aberrations (CAs)

Lateral chromatic aberrations (color shadows at harsh contrast transitions) can
be a problem at 20mm. The average CA width at the borders varies between 1.76px and
2.45px - this can be visible in field images. The problem is much better controlled
at the 35mm setting.

Note: Lateral CAs can be reduced via imaging tools.

Verdict

The Pentax SMC-FA 20-35mm f/4 AL is a very good lens but it doesn't really offer much
over its successor - the SMC DA 16-45mm f/4 ED AL. Similar to its cousin it
has one primary problem: lateral CAs (color shadows at harsh contrast transitions)
at 20mm. Other than that the lens is capable to produce a very good
resolution and comparatively low distortions and vignetting. The build quality is
about average for a consumer grade zoom. The lens had its days during the film
era but it's time to move on now.